There are a number of different types of circumstances (or applications) wherein a rotatable bit is used to engage a substrate (e.g., earth strata). These different circumstances use assemblies such as a rotary drum, a cutting chain, a scrapper (or scarifier) blade, and drilling equipment (e.g., augers). Examples of these applications include underground mining, surface mining, dredging, construction road planning, trenching, road grading, and snow/ice removal.
In some of the above applications, the rotatable bit is subjected to an intermittent type of engagement (or cutting) of the substrate. For example, when a rotatable bit is mounted to a rotary drum of a road planning machine, the bit cycles in and out of engagement with the substrate (e.g., the cut) as the drum rotates. The same is true with respect to a rotatable bit used on a cutter chain. Even in scrapper blade applications the rotatable bit experiences an intermittent engagement (or cutting action) with the earth strata due to the fact that the rotatable bits periodically are not in engagement with the earth strata when the blade passes over low areas of the substrate surface.
In intermittent engagement applications, such as, for example, road planning, at any one time there are a number of the rotatable bits that are not in engagement with the substrate. When the rotatable bits that are in engagement with the substrate encounter an obstacle (e.g. a man hole cover) the rotational speed of the drum quickly reduces so that the forces exerted on the rotatable bit operate so as to try to eject the rotatable bit from the bore of the holder. Unless there is sufficient force to retain the rotatable bit in its holder, these forces will eject the rotatable bit from the bore of its holder. In order to maintain the rotatable bit in its holder, it has been necessary to use a retainer that has radially outwardly projecting dimples. This type of arrangement is along the lines of the block-rotatable tool combination disclosed in U.S. Pat. No. 3,752,515 to Oaks et al.
In the arrangement such as disclosed in U.S. Pat. No. 3,752,515, the bore has an annular groove near the rearward end of the bore. Each rotatable bit has a rearward shank that contains a reduced diameter portion that carries a resilient retainer ring that has a plurality of radial outwardly projecting dimples. When the rotatable bit is inserted into the bore, the resilient retainer ring is compressed as the drilling bit slides (or is forced) into the bore. At the point wherein the dimples register with the annular groove, the resilient retainer ring expands so that the dimples are received within the annular groove. The reception (or engagement) of the dimples by the annular groove retains the rotatable bit within the bore of the holder during the operation of the assembly.
Another arrangement designed to retain a rotatable bit in a bore of a holder is disclosed in U.S. Pat. No. 4,201,421 to Den Besten et al. In this arrangement, the long resilient retainer sleeve extends for a substantial length of the shank of the rotatable bit. The fact that the sleeve extends along a substantial length of the shank provides for sufficient force to retain the rotatable bit in the bore of the holder. The long resilient retainer sleeve has been used on rotatable bits employed in road planning applications.
In the past, drilling assemblies have been used to drill holes in substrates such as earth strata. One version of such a drilling assembly is a bullet tooth rock auger sold by Reedrill, a division of Metso Minerals of Sherman, Tex., as a Texoma® auger tool. This bullet tooth rock auger comprises an auger that includes a central shaft with a helical auger flight affixed to the central shaft. A plurality of holders are affixed to the distal end of the central shaft. The helical auger flight has a peripheral edge. Holders are affixed to the lower portion of the helical auger flight at the peripheral edge thereof.
In a drilling operation, the rotatable bit is in constant contact (or engagement) with the substrate (e.g., earth strata). As a result, there is not the sudden force exerted on the rotatable bit to eject the bit from the holder. This means that there is not the same requirement for a retainer that retains the rotatable bit in the bore of the holder against a sudden ejecting force. However, even though the requirement for the retaining force is not as great in a drilling application as other applications, heretofore, rotatable bits used in drilling applications (i.e., drilling bits) still use a retainer such as in U.S. Pat. No. 3,752,515 to Oaks et al.
To eject the drilling bit from the bore of the holder, the operator strikes the rear end of the drilling bit. Such an impact forces the drilling bit from the bore of the holder. It is intended that upon striking the drilling bit, the resilient retainer ring will be compressed so that there no longer is any engagement of the dimples by the annular groove. The drilling bit then can be forced out of the bore by additional impacts on the rearward end of the drilling bit.
During the drilling operation, there is a build up of dirt and other debris in the bore of the holder. Some of this debris collects between the resilient retainer ring and the reduced diameter portion of the drilling bit. The presence of this debris between the resilient retainer ring and the reduced diameter portion of the drilling bit obstructs the compression of the retainer ring. This makes it difficult for the retainer ring to compress when the operator strikes the rear end of the drilling bit. In those cases where the resilient retainer ring does not fully compress, the dimples must be sheared off in order for the drilling bit to be extracted from the bore of the holder. As can be appreciated, it can sometimes take a lot of effort and time to remove drilling bits from their corresponding holders if the dimples have to be sheared off to remove the drilling bit because the resilient ring experiences difficulty compressing due to the presence of the debris between the resilient retainer ring and the reduced diameter portion of the drilling bit. An increase in the time and effort needed to remove the drilling bits increases the overall cost of a drilling operation.
It would very desirable to provide an improved drilling bit that is not susceptible to difficulties associated with the built up (or accumulation) of debris between the resilient retainer ring and the reduced diameter portion of the drilling bit. More specifically, it would be desirable to provide an improved drilling bit wherein the resilient retainer is not susceptible to an inability to compress due to the collection of dirt and debris between the retainer and the reduced diameter portion of the drilling bit.